1. Field of the Invention
The present invention relates to a vibration isolation system, vibration isolation method, a lithographic apparatus and a device manufacturing method. The present invention also relates to vibration isolation using modal decoupling.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one, or several, dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a prior art lithographic apparatus, a projection system for projecting a patterned beam onto a target portion of the substrate is supported by and positioned relative to a metrology frame. The metrology frame is supported by and positioned relative to a base frame.
Vibrations of the projection system may cause incorrect projections on the substrate rendering the substrate unusable. Therefore, any vibration of the projection system is to be prevented. The base frame however is coupled to the environment of the lithographic apparatus, such as a factory floor. The base frame passes any vibration of the environment on to any body rigidly coupled to the base frame. The metrology frame is not rigidly coupled to the base frame, but is coupled and supported using springs, preferably air springs, which isolate and damp certain vibrations. Other vibrations need to be isolated and damped by active isolator devices.
In the prior art lithographic apparatus, active devices are used in combination with the air springs, which are passive devices, although it is also known to use pneumatically controlled air springs, in which case the air springs are active devices for low frequencies, but may be regarded as passive devices for higher frequencies. Essentially, the active isolator devices may be regarded as active for frequencies where the air springs are or may be regarded as passive.
In the prior art lithographic apparatus, sensors detect any vibration of the metrology frame and the detected vibration is fed to a control system. In response to the detected vibrations, the control system determines a compensation to be performed by the active isolator devices. The compensation is intended to isolate and damp the detected vibration. The compensation also be employed to position the metrology frame with respect to the base frame.
The metrology frame has six degrees of freedom: translations in three directions and rotations in three directions. This implies that a vibration may be decomposed in those six (Cartesian or other orthogonal) directions and a vibration may be isolated and damped by compensations in those six directions. However, a compensation force in one direction may result in a movement not only in the one direction, but also in one or more of the other five directions. Thus, the control system needs to be a multiple-input multiple-output (MIMO) system. Such a system is a complex system, in particular if the system is unstable in at least one direction.
If the system is unstable in one direction, it needs to be stabilized by the control system, since a force exerted on the system in such an unstable direction may lead to an uncontrollable movement in the unstable direction, and may even lead to damage to the system. Generally, in a MIMO system, such instability and corresponding stabilization in one direction results in forces and vibrations in other directions, since the directions are coupled. Thus, an isolation and damping performance in a coupled other direction is compromised when the unstable direction is stabilized.